The present invention relates to an oven, in particular, an oven for dental material.
A so called retort oven is commonly used for the production of dental ceramic as well as for other purposes within the dental area. In this connection, a basic configuration which has shown itself to be particularly practical includes the actual heating component integrated into a liftable or swing away oven hood and comprises a substantially planar working surface having thereon corresponding markings for indicating the location at which a retort (24) for the curing of the ceramic is to be placed. This configuration permits a rapid work cycle and a minimized shaking movement of the retort (24) having the dental ceramic therein.
Retort ovens of the type just described have basically proven themselves and have been in service since easily more than a decade. However, such retort ovens must be regularly calibrated in order to assure uniform quality of the item to be fired. In this regard, a calibration program has typically been implemented. This program includes disposing a test support supporting a lengthwise extending silver wire on the curable item surface and performing a calibration sequence. An indication of a correct calibration sequence is the formation of a small, downwardly hanging bead at the end of the horizontally extending wire. A failed calibration results in either the melting of the calibration wire or an absence of melting of the wire, whereby, after an appropriate pause, a repeat of the calibration sequence must be performed. Although the melting point of gold, for example, isxe2x80x94at 1064.76 degrees C.xe2x80x94precisely ascertainable, the calibration approach just described can only yield a precision which is no more precise than within a few degrees.
Moreover, it has been suggested to wind a silver wire between two pins or posts which are disposed in sockets. An interruption of the electrical connection established through the silver wire is thus detectable upon melting of the wire. However, this approach is disadvantageous for a number of reasons. The winding does not assuredly offer the possibility of an established wire contact for the reason that the contact pressure is highly dependent upon the individually variable winding application applied by the user. There also exists the risk that oxidation will preclude establishing a reliable contact with a contact post. Additionally, a wound silver wire is prone to expansion as a result of its heating up, a result, in fact, to be expected in view of the thermal expansion coefficient of silver. It is, however, for precisely this reason that there arises the risk that such expansion will cause at, for example, 700 degrees, the breaking off of a contact which otherwise exists at room temperature. In this event, an electronic element disposed for detecting such interruptions of contact will erroneously deem such an interruption of the circuit as an indication that a desired temperature has been reached, thereby creating a significantly false calibration result.
A further disadvantage is the melting silver wire dirties the socket in which it is disposed, thereby leading to a risk of current leaking.
Yet a further disadvantage is that the cable for the calibration apparatus must be conducted to the exterior. This cable is effectively clamped, whereby it is subjected to significant stress. Moreover, it is in reality no longer possible to introduce a vacuum in the oven as the need may arise, for the reason that exterior ambient air flows interiorly along the slots of substantial length extending on both sides of the cable to such an extent that even a powerful vacuum pump cannot effect an effective reduction of the inner pressure.
Outside of the dental material oven environment, numerous other possibilities are known for calibrating an oven. For example, a calibration apparatus is disclosed in U.S. Pat. No. 5,331,676 which provides thermal elements in spaced relation to one another. It is further known, as disclosed in German patent document DE-OS 42 18 032, to provide an oven for thermal analysis, in which a test support with two thermal elements are disposed, the energization current for the thermal elements extending through the underside of the oven. The test support rests in freely movable disposition of the top surface of the underside of the oven so that the test support can, in effect, slide or slip. This calibration approach thus attempts to exploit the fact that the oven interior, although substantially wide and long, is flat.
The known approaches to calibration accordingly suffer from the significant disadvantages that an insecure and/or imprecise calibration risk is produced when such calibration approaches are implemented in connection with retort ovens.
It is one object of the present invention to provide an oven for dental material which can be calibrated in a reliable yet economical manner.
This and other objects of the present invention are achieved by an oven for dental material having the features recited in the claims herein.
The oven of the present invention comprises a firing plate, and a calibration apparatus removably locatable on the firing plate, the calibration apparatus including a meltable element which melts upon heating thereof to a predetermined temperature such that the melting of the meltable element effects a change of condition detectable by a condition change detecting device, a pair of electrically conducting elements for supporting the meltable element therebetween, and a non-electrically conducting support for supporting the pair of electrically conducting elements.
One particular advantageous configuration of the oven for dental material in accordance with the present invention includes an assembly provided as a central element on the firing plate, this assembly being covered in normal operation of the retort oven by a full covering and being uncovered for calibration. This assembly, which is preferably configured as a pan or an element having a recess, precisely establishes the position of the calibration apparatus whose outer dimensions are selected in precise correspondence with the pan or the element having a recess. In this way, it can be ensured that a false correlation or reconciliation does not result from a false placement of the calibration apparatus.
The pan permits the covered extension of the contacts which further permits the avoidance of the disadvantage that would otherwise arise from the extension of cables under an oven head. In this connection, the sealing of the oven, especially, is so configured that a vacuum operation during calibration can be implemented, if needed. This, on the other hand, opens the possibility as well that, with a view on the effect of oxidation, less critical material for the electrical conductor need only be used. No gold or platinum need be used in order to keep the calibration wire temperature stable and scale free, and thus contact secure. This makes possible, on the other hand, the use of gold calibration wire for an improved calibration, which melts at 1,064.76 degrees C. and to that extent affords an exact and improved reference.
It is also possible, through sequential use of gold and silver calibration wire, to use two calibration points, in order as well to compensate for the always necessary disadjustment of the oven in view of the temperature course and to effect the post calibration of the oven.
Through the possibility to drastically reduce the oxidation tendency by creation of a vacuum, there exists on the other hand, the possibility of configuring a reusable calibration apparatus capable of many cycles of use. In this manner, the need is eliminated to configure the calibration apparatus as a one-time use or disposable article (with the associated problems of reduced quality or, respectively, contact problems).
In accordance with the present invention, the calibration wire is preferably disposed in tension between the contact posts. In this manner, a secure contact disposition is assured and, especially, the risk is avoided that a secure contact cannot be assured due to a thermal expansion of the calibration wire. The disposition of the calibration wire in tension between the contact posts is preferably implemented by configuring the contact post as a V-shape recessed element (54) at one end thereof, in which the calibration wire is frictionally retained.
It is particularly advantageous to extend the contact connections through the floor of the oven, namely, through the stone footing, such that the contact connections are protected. In this way, it can be reliably insured that the cable is not pinched or clamped by the heavy oven hood.
In one advantageous configuration of the present invention, it is provided that contact through the stone footing is via genuinely thin wire. Such wires act as only negligible thermal bridges, at least in the operation of the pan in which they terminate, which is covered by a full plate. According to one variation, it is provided, in contrast, that the stone footing is configured with genuinely thin bores which receive therein the contact posts of the calibration apparatus upon its installation with the contact posts being received in contact springs in the bores. This configuration also permits the maintenance of a vacuum during the calibration operation.
In accordance with another variation of the calibration apparatus of the present invention, the calibration wire is omitted in lieu of a thermal element which registers a resistance change upon the achievement of a predetermined temperature which is taken into account during calibration.
In an advantageous configuration of the calibration apparatus of the present invention, the adjustment of the retort oven occurs automatically. The substantial resistance change due to, for example, the melting of the calibration wire, is captured and used as a reference temperature for the compensation of the oven.
Further details, advantages, and features are described in the following description of the several variations of the present invention in connection with the drawings.